We have demonstrated that the effects of castration and steroid treatment on pituitary gonadotrpin release and sexual behavior in the hamster are dependent upon the light:dark cycle the animals are exposed to. These results suggest that light can induce fundamental changes in the brain's sensitivity to steroid hormones. In the present proposal we plan to examine in more detail how the length of the day can alter the sensitivity of the hypothalamic-pituitary system to steroid feedback. In one set of studies we will determine the temporal sequence of changes which occur in steroid sensitivity during transfer from long to short and short to long days. These data will be compared with the temporal sequence of changes which occur in gonadotropin release in intact, untreated animals during transfer from one photoperiod to another. The results from these studies may enable us to determine if light-induced changes in steroid sensitivity play a major role in the regulation of the breeding season in seasonally breeding mammals. Since the pineal gland is known to be involved in the transfer of information about changes in the photic environment into changes in the new neuroendocrine-gonadal axis, we plan to assess the involvement of the pineal gland in light-induced changes in the sensitivity of the hypothalamic-pituitary axis to steroid hormones. By integrating the information obtained from the proposed studies, we hope to develop a better understanding of how the photoperiod, gonadal secretions and the pineal gland are involved in the regulation of neuroendocrine-gonadal activity. BIBLIOGRAPHIC REFERENCES: Turek, F.W. Role of the photoperiod in altering the response of the hypothalamic-hypophyseal axis to the inhibitory effects of testosterone. Fed. Proc. 36: 321, 1977 (abstract). Turek, F.W. Antigonadal effects of melatonin in pinealectomized and intact male hamsters. Proc. Soc. Exp. Biol. Med., 1977, in press.